1. Field of the Invention
The present invention relates generally to voltage regulators, and more particularly to a voltage regulator having the function of preventing reverse current flowing from the output side to the input side and protecting the circuit at the time of reverse connection of a direct current power supply where the direct current power supply is connected in reverse polarity to the voltage regulator.
2. Description of the Related Art
In conventional voltage regulars where circuits are formed of CMOS transistors, circuit failure may be caused by a large forward current caused to flow through a pn junction in a MOS transistor by reverse current flowing from the output side to the input side when a direct current (DC) power supply is connected in reverse polarity or an output voltage is greater than an input voltage. Therefore, conventionally, a PMOS transistor M102 is disposed between an input terminal IN and an output transistor M101 as shown in FIG. 1 so that the PMOS transistor M102 is turned OFF to be non-conducting when an output voltage VOUT is greater than an input voltage VIN or a DC power supply is connected in reverse polarity to the voltage regulator, thereby preventing circuit failure.
Although different from the present invention, there has been provided a reverse battery connection protection circuit that prevents damage to a semiconductor switching device in the case of reverse connection of a battery (for example, Patent Document 1 listed below). Further, there has also been provided a step-down switching regulator having a changeover switch to switch connections at the substrate gate of a switching transistor so as to be capable of preventing backflow of current without using a diode for backflow prevention even in the case of using a PMOS transistor for the switching device of the switching regulator (for example, Patent Document 2 listed below).
[Patent Document 1] Japanese Laid-Open Patent Application No. 2005-137190
[Patent Document 2] Japanese Laid-Open Patent Application No. 2006-34033
In the case of FIG. 1, however, there is a problem in that if the output transistor M101 is increased in size in order to increase output current, which is an index of the performance of the voltage regulator, the PMOS transistor M102 on the input terminal side should be greater in current driving capability than the output transistor M101, which runs against product downsizing. Further, there is another problem in that the resistance at the time of operation increases because the PMOS transistor M102 is connected in series to the output transistor M101.